In the past, direct converter circuits, such as matrix converters, tended to be of academic importance. Today, however, direct converter circuits are gaining in importance, particularly for industrial applications, because an input voltage or an input current of a first amplitude and a first frequency can be converted directly into an output voltage or into an output current of a second amplitude and a second frequency by means of a direct converter circuit with no expensive intermediate direct voltage circuit or intermediate direct current circuit. A direct converter circuit of this type is disclosed, for example, in U.S. Pat. No. 6,900,998 B2. Here, the direct converter circuit has n=3 input phase connections and p=3 output phase connections, i.e. the direct converter circuit from U.S. Pat. No. 6,900,998 B2 has a three-phase design on the input side and on the output side. The direct converter circuit from U.S. Pat. No. 6,900,998 B2 furthermore includes nine phase modules, each with a two-pole switching cell to switch a positive and a negative voltage between the poles, wherein each output phase connection is connected in series with each input phase connection in each case directly via a switching cell. A switching cell of this type has controllable two-way power semiconductor switches with a controlled one-way current-carrying direction and a capacitive energy store.
A drawback with a direct converter circuit according to U.S. Pat. No. 6,900,998 B2 is that the voltage on each branch, i.e. on each phase module, cannot be set in such a way that a continuous current flow can be achieved by the switching cells, as a result of which no active current setting by the respective branch is possible. As a result, only a very restricted or no exchange of electrical energy between individual branches is possible with the direct converter circuit from U.S. Pat. No. 6,900,998 B2. However, if the direct converter circuit is intended to be capable of transferring a large amount of electrical energy, the capacitors of the switching cells from U.S. Pat. No. 6,900,998 B2 must be dimensioned as correspondingly large, resulting in an enormous space requirement of a direct converter circuit of this type and considerable costs. As a result, systems set up with direct converter circuits of this type will similarly have correspondingly substantial space requirements and will be correspondingly expensive.
Energy fluctuations in the individual phase modules result in voltage fluctuations in the capacitive energy stores in the associated switching cells. However, for a reliable and stable operation, and in order to enable a low-cost implementation of the direct converter circuit, it must be possible to limit and minimize the amplitude of this energy fluctuation on the phase module in order to be able to limit the maximum voltage on each individual capacitive energy store of the associated switching cell of the phase module to a required value with the smallest possible capacitive energy store.
In “A Methodology for Developing ‘Chainlink’ Converters”, EPE 8 Sep. 2009, a direct converter circuit is disclosed in which each phase module has an inductor in series with the series circuit of the switching cells.
In WO 2008/067788 A1, a method is disclosed for the operation of a converter circuit according to WO 2007/023064 A1, which regulates the energy content of the switching cells. The method described in WO 2008/067788 A1 applies only to designs of the converter circuit according to WO 2007/023064 A1 which connect three phases of one system to two phases of another system, wherein the currents in the connection terminals of the direct converter circuits are always zero.
In “On Dynamics and Voltage Control of the Modular Multilevel Converter”, EPE 8 Sep. 2009, a method is disclosed for the operation of a converter circuit, in which the balancing of the phase modules is effected with the aid of a control provided specifically for that purpose.